This invention relates to an optical recording medium for making optical read and write of information by using an organic dye.
Photochromic materials are attracting attention for their availability for the erasable optical recording media owing to their specific property that they undergo a reversible change of color upon exposure to two types of light source differing in wavelength. Fulgides are known as a typical example of such photochromic materials. Fulgides are the compounds having an alkylidene group bonded to each of the two methylenic carbons of succinic anhydride or a derivative thereof, and represented by the following general formula (1): ##STR1## (wherein at least one of R.sub.2 and R.sub.3 represents an aromatic ring, and X represents 0 or N--R).
A large number of fulgide compounds have been known to date. Among them, fulgide (2) is one of the fulgides having the most excellent photochromic properties (JCS Perkin Trans., Part I, 202 (1981). This fulgide (2), when irradiated with ultraviolet light of 337 nm, is ring closed and converted into the red benzofuran form (3), but when the latter is irradiated with visible light of 473 nm, it returns to fulgide (2). ##STR2##
In utilization of photochromic compounds for optical recording media, light quantum detector elements, etc., it is required to reduce the film thickness in correspondence to the miniaturization in size of electronic parts. For forming a uniform ultra-thin Langmuir-Blodgett film of an organic compound, it is necessary that a hydrophobic group (for example, hydrocarbon chain) and a hydrophilic group (for example, carbonyl group) be contained in the molecule of said compound. However, since the conventional fulgide (2) has no hydrophobic group in the molecule, it has been impossible to obtain a fulgide film of a desired small thickness by the Langmuir-Blodgett technique. Also, no successful attempt of direct introduction of a hydrophobic group into the molecule of fulgide (2) has yet been reported.